[2021] COMP1411 Introduction to Computer Systems - Q4 Instruction Execution Performance
Question 4. [22 marks]
Given the sequence of assembly instructions (in the Y86-64 instruction set).
irmovq $-1, %rax
xorq %rax, %rax
rmmovq %rbx, 10(%rcx)
rmmovq %rax, 20(%rcx)
jg .L1
rrmovq %rbx, %rax
ret
L1:
rmmovq %rax, (%rcx)
shalq $4, %rax
ret
The execution of each instruction will go through 6 stages: Fetch (F), Decode (D), Execute (E), Memory (M), Write-back (W), and PC-update (P). Any stage of a specific instruction can be executed only after the same stage of its immediately preceding instruction is finished. Assume there is no extra stalling for any stage of any instruction, and no instruction or data are cached.
If one instruction has operation in a stage, executing the stage takes some time; if otherwise the instruction has no operation in a stage, it still stays in this stage for a small amount of time, during which the next instruction cannot enter this stage. Assume that the instruction “ret” takes 0 time to execute. Assume the first instruction starts at time 0.
The time consumptions are listed in the following table.
Question 4. [22 marks]
Given the sequence of assembly instructions (in the Y86-64 instruction set).
irmovq $-1, %rax
xorq %rax, %rax
rmmovq %rbx, 10(%rcx)
rmmovq %rax, 20(%rcx)
jg .L1
rrmovq %rbx, %rax
ret
L1:
rmmovq %rax, (%rcx)
shalq $4, %rax
ret
The execution of each instruction will go through 6 stages: Fetch (F), Decode (D), Execute (E), Memory (M), Write-back (W), and PC-update (P). Any stage of a specific instruction can be executed only after the same stage of its immediately preceding instruction is finished. Assume there is no extra stalling for any stage of any instruction, and no instruction or data are cached.
If one instruction has operation in a stage, executing the stage takes some time; if otherwise the instruction has no operation in a stage, it still stays in this stage for a small amount of time, during which the next instruction cannot enter this stage. Assume that the instruction “ret” takes 0 time to execute. Assume the first instruction starts at time 0.
The time consumptions are listed in the following table.
Stages | F | D | E | M | W | P |
Has operation | 10 | 2 | 2 | 12 | 2 | 1 |
No operation | 1 | 1 | 1 | 1 | 1 | 1 |
4(a) Compute the starting and end time of each executed instruction, and the time spent in each stage for each instruction. [16 marks]
Show details on your reasoning about the behavior of the instructions.
4(b) Identify and explain the main bottlenecks for the performance of instruction execution, and provide some technique that can be used to improve the performance.
[6 marks]
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